Conventionally, as a means for monitoring the rearward travel of an automobile, a mirror or a video camera is installed to permit visual monitoring for obstacles, and therefore, the driver could only watch the rear direction when driving the automobile in the reverse direction. Accordingly, it has been impossible to perform a parking at an exact position, or to prevent the approaching of another car from the rear. However, recently non-visual, methods i.e., infrared ray detector or ultrasonic detector have been developed and put to the practical use to monitor objects at the rear of a vehicle.
However, the infrared detector is not capable of precisely determining the distance of the object due to the color sensitivity of the infrared rays, thereby making it impossible to perform an exact distance detection when parking an automobile. That is, when infrared rays are reflected from a primary color and a black color respectively, a considerable error is generated, so that an exact measurement should be rendered impossible.
Meanwhile, in the case of the ultrasonic detector, although measurement error arising from different colors can be eliminated, its detecting range is very limited, because the ultrasonic apparatus uses a beam type ultrasonic waves. Consequently, its use is limited to only parkings. Further, there is another disadvantage such that a plurality of ultrasonic apparatuses have to be installed if the reliability is to be assured. Further, there are other disadvantages such that: its automatic gain control (AGC) is limited so that a single device cannot measure both near and far objects the apparatus is subject to malfunction in winter seasons due to the lack of a temperature compensation in the circuit resulting in faulty circuit operation.
Further, in this ultrasonic method, the distance of an object is measured based on the reflectivity by measuring the reflected amount of the ultrasonic waves, with the result that its measuring range is very narrow, and its measuring precision is very unreliable.